Note Synthetic study of 3-fluorinated sialic acid derivatives Katsuhiko Suzuki a, * , Shusaku Daikoku a , Sang-Hyun Son a , Yukishige Ito a, b , Osamu Kanie a, c, * a ERATO, Japan Science and Technology Agency (JST), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b Synthetic Cellular Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan c Institute of Glycoscience, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan article info Article history: Received 16 October 2014 Received in revised form 22 December 2014 Accepted 23 December 2014 Available online 14 January 2015 Keywords: Alkylation Glycosides Inhibitor Substrate abstract Sialic acid derivatives, analogs, and their conjugates are expected to be pharmaceutical candidates such as anti-influenza drugs and also useful probes for investigating the biological role of glycoconjugates. Derivatives of 3-fluorinated sialic acid (3-F-Sia) have been found to be excellent probes in investigating functions and mechanisms of a series of proteins. Here, we describe the syntheses of 3-F-Sia derivatives, which are useful in making biologically important conjugate probes. A practical method for the con- struction of 3-fluorinated sialosides based on the stereoselective formation of the corresponding anomeric O-trimethylsilyl ether and their nucleophilic attack by an alkyl halide, an allyl halide in particular, was developed. In addition, details of the synthesis of cytidine monophosphate (CMP)-3-F-Sia bearing a fluorescent tag, which has been proven to show dual functions as a substrate of CMP-sialic acid transporter (CST) and an inhibitor of sialyltransferase (STase), are described. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Sialic acid, a member of the carbohydrate family, is often found in glycoproteins and glycolipids, some of which are known to be involved in important biological functions such as tumor metas- tasis, immune response, infection, and brain function. In fact, the analogs and derivatives of sialic acid have been shown to be potent inhibitors of influenza virus sialidase, and they are commercially available. 1,2 Moreover, sialic acid derivatives exhibiting immuno- regulatory activity have been reported. 3 Given the important functions of a glycan-bearing sialic acid, its derivatives and analogs are expected to become valuable tools for investigating the bio- logical roles of glycoconjugates. 3-Fluorinated sialic acid (3-F-Sia) is one of the simplest analogs, 4 whose glycoside is known to be resistant to both acidic hydrolysis and sialidase-catalyzed hydrolysis. 5 It is expected that introducing a fluorine atom at a particular position of a- and b-glycosides may stabilize the glycosidic linkage owing to the destabilization of the oxocarbenium intermediate formed during the hydrolysis reaction. For instance, the p-nitrophenyl a-glycoside of 3-F-Sia was reported to be an inhibitor of sialidase. 5 Furthermore, it was reported that an alkyl a-glycoside of 3-F-Sia bearing dis- tearoylphosphatidylethanolamine inhibited influenza sialidase and hemagglutinin. 6 The corresponding cytidine monophosphate (CMP)-sialic acid analog was found to be an inhibitor of sialyltransferase (STase). 7e9 These facts suggest that 3-F-Sia derivatives are promising modu- lators of important biological events in which sialic-acid-related enzymes are involved. In this respect, several groups have recently reported interesting results using 3-F-Sia derivatives. 10e12 CMP-sialic acid is synthesized in the nucleus unlike other sugar nucleotides, and it is transported through the nuclear pore to the cytoplasm and then into the Golgi apparatus via the CMP-sialic acid transporter (CST); however, the mechanisms of these events are not understood. 13 We expect that a chemical probe based on the structure of CMP-sialic acid would be a valuable tool for live-cell imaging owing to the advantage of its specific ability to be trans- ported into Golgi vesicles. In this respect, a fluorescently tagged CMP-sialic acid derivative has been used previously to study the synthesis of sialylated glycosphingolipids. 14 The presence of a fluorine atom at the C-3 position of sialic acid was assumed to be tolerated by the CST because it mainly recognizes the nucleoside part of CMP-sialic acid. 15 A fluorescently labeled CMP-3 00 -F-Sia de- rivative (1) is, therefore, effective in visualizing Golgi vesicles, by virtue of selective transportation by the CST while inhibiting sia- lylation (Fig. 1). On the basis of these considerations, we reported * Corresponding authors. Tel.: þ81 463 58 1211x4656; fax: þ81 463 50 2432. E-mail address: kanie@tokai-u.jp (O. Kanie). Contents lists available at ScienceDirect Carbohydrate Research journal homepage: www.elsevier.com/locate/carres http://dx.doi.org/10.1016/j.carres.2014.12.010 0008-6215/© 2015 Elsevier Ltd. All rights reserved. Carbohydrate Research 406 (2015) 1e9